The invention relates to a roof structure wherein a plurality of purlins, and particularly Z-purlins, are disposed across the rafters and secured thereto to constitute a secondary frame supported on the primary frame provided by the rafters which may be H-beams, I-beams or channel sections, or one of the various types of open webbed steel joists, girders or trusses. The Z-purlins constitute the secondary frame and they support directly the roof deck which may consist of steel interlocking panels. Examples of panel structures may be found in copending application Ser. No. 761,479 filed Jan. 24, 1977 in the name of Donald M. Taylor et al U.S. Pat. No. 4,102,105.
the Z-purlins of the secondary frame are subject to premature failure by rotation and lateral deflection and by web buckling which is caused by vertical loading, as for example snow loads. Because of the Z configuration, a failure by roll-over invariably occurs in a sense which would tend to open up the Z, and web crippling failure can develop at bearing points on interior rafters. It has been the practice with Z-purlins to have them placed facing in the same direction. Thus, for example, in a simple slope roof the Z cross sections of the purlins all face up the roof slope. For a double sloped roof, all purlins on each roof section are disposed to face upward toward the ridge.